![]() ![]() The recent discoveries of light-induced metallicity 16 and intercalation-controlled magnetism 17 provide complementary pathways for property regulation, although the structural aspects of these processes are under-explored. Whether this is the case in NiPS 3 is not yet apparent, but it is already clear that this system has a rich set of symmetry progressions 12 as well as metallic 11, 12, 15 and relaxation behavior 15 making it an intriguing platform for deeper study. As demonstrated in FePS 3 and CrSiTe 3, a low-temperature structural phase transition triggers superconductivity as well 2, 14. Compression thus gives rise to several new states of matter in the MPS 3’s ( M = Mn, Ni, Co, V)-each with properties that can be deterministically controlled. Instead, this system hosts an entire series of structural phase transitions and associated symmetry progressions of mysterious origin. A similar progression might be anticipated to take place in NiPS 3 6, 10, 11, 12, 13, but even the most cursory inspection shows that this is not the case. ![]() As demonstrated in FePS 3 as well as the Mn and V analogs, external stimuli such as pressure (and likely strain) induce a layer-sliding transition and systematic bandgap reduction followed by a volume collapse to the metallic state at room temperature 1, 2, 3, 4, 5, 6, 7, 8, 9. Structural phase transitions in complex chalcogenides are attracting considerable attention as drivers of exciting new states of matter with the potential to host emergent properties. In addition to identifying a candidate polar metal ripe for further inquiry, we suggest that pressure may tune other complex chalcogenides into this elusive state. Even more strikingly, infrared spectroscopy and X-ray diffraction combined with a symmetry analysis reveal both metallicity and loss of the inversion center above ~23 GPa suggesting that NiPS 3 may be a polar metal with a P3 m1 space group under these conditions and P1 symmetry under maximum compression. ![]() Remarkably, we find five different states of matter between ambient conditions and 39 GPa-quite different than in the other MPS 3 materials. In this work, we bring together diamond anvil cell techniques, infrared and Raman scattering spectroscopies, and X-ray diffraction with a detailed symmetry analysis and first-principles calculations to uncover a series of high-pressure phases in NiPS 3. Under pressure, complex chalcogenides like MPS 3 ( M = Mn, Ni, Co, V) host sliding and structural transitions, insulator-to-metal transitions, the possibility of an orbitally-selective Mott state, piezochromism, and superconductivity. Van der Waals solids are ideal platforms for the discovery of new states of matter and emergent properties under external stimuli. ![]()
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